Electrophotography is a useful process for printing images on a receiver (or “imaging substrate”), such as a piece or sheet of paper or another planar medium, glass, fabric, metal, or other objects as will be described below. In this process, an electrostatic latent image is formed on a photoreceptor by uniformly charging the photoreceptor and then discharging selected areas of the uniform charge to yield an electrostatic charge pattern corresponding to the desired image (a “latent image”). The photoreceptor retains the latent image, e.g., on its surface or under a protective ceramic overcoat.
After the latent image is formed, charged toner particles are brought into the vicinity of the photoreceptor and are attracted to the latent image to develop the latent image into a visible image. Note that the visible image may not be visible to the naked eye depending on the composition of the toner particles (e.g., clear toner).
After the latent image is developed into a visible image on the photoreceptor, a suitable receiver is brought into juxtaposition with the visible image. A suitable electric field is applied to transfer the toner particles of the visible image to the receiver to form the desired print image on the receiver. The imaging process is typically repeated many times with reusable photoreceptors.
The receiver is then removed from its operative association with the photoreceptor and subjected to heat or pressure to permanently fix (“fuse”) the print image to the receiver. Plural print images, e.g., of separations of different colors, are overlaid on one receiver before fusing to form a multi-color print image on the receiver.
FIG. 2 shows an embodiment of a typical toning station 23 for bringing toner particles into the vicinity of photoreceptor 25. This station uses two-component developer, also called multi-component developer. Developer 235 includes toner particles and magnetic carrier particles. Belt photoreceptor 25 is adjacent to toning station 23, which includes toning roller 210, feed roller 220, and sump 230. Sump 230 contains developer 235. Feed roller 220 includes protrusions 225 for carrying developer from sump 230 to toning roller 210. Metering skive 214 is spaced apart from toning roller 210 to permit an appropriate amount of developer to pass to toning zone 217, in which toner is transported to photoreceptor 25. Mixer 237 rotates to tribocharge and mix developer 235. An example of a system using a feed roller and mixing augers is given in U.S. Pat. No. 7,792,467, the disclosure of which is incorporated herein by reference.
Toner particles are attracted to magnetic carrier particles by electrostatic forces developed by tribocharging in sump 230. As toner particles and carrier particles are moved against each other by mixer 237, they develop opposite charges and are thus attracted to each other. The toning station 23 brings developer into proximity with the latent image on the photoreceptor. A magnetic field is applied to the magnetic carrier particles to cause them to lift towards photoreceptor 25. The toner particles attracted to the carrier particles are thus brought closer to the latent image. This reduces the electrostatic force required to transfer toner particles from the developer to the latent image, and thus provides a visible image which more completely fills the toner areas of the latent image.
Various schemes have been proposed for mixing toner and carrier particles to provide effective development, especially when fresh toner is added to replace toner that has been transferred to the photoreceptor (“toner replenishment”). The above-referenced '467 patent uses augers in the sump to mix developer. Other systems add fresh toner to the end of a return channel where depleted developer empties into a sump or auger racetrack. However, this can lead to dusting, a phenomenon in which relatively uncharged or relatively low-charged toner particles become airborne due to the high kinetic energy imparted to them by mixer 237, e.g., a mixing auger or paddle.
In some toning stations, replenishment toner is supplied to the toning station 23 through tube 238 that has either holes or slots. This tube 238 is usually disposed over the sump 230, and an appropriate amount of replenishment toner 239 is dropped into the toning station sump 230 at the point where toner-carrier mixing occurs. As this mixing area is usually diffusive, a considerable amount of agitation of the developer may be present in the toning station sump 230. The action of dropping the toner onto the surface of the toning station sump 230 can cause undesired effects. Toner, when released from the replenisher tube 238, can be moved by any air currents within the toning station 23 and deposited on unwanted locations within the toning station 23. The toner can also travel out of the toning station 23, depositing dust on other subsystems in the electrophotographic module and reducing image quality.
There is a need, therefore, for an improved way of replenishing toner in a multi-component dry electrophotographic printer.